In a communication system, frequency and phase offsets always exist between the local oscillators of the transmitting and receiving sides. In a Quadrature Amplitude Modulation (QAM) system, the carrier offset induces rotation and tilt to the signal constellation, and such situation destroys the signal severely.
Please refer to FIGS. 1A˜1C which schematically show three types of signal constellation plots of demodulated QAM-16 signals, respectively, wherein FIG. 1A shows an ideal signal constellation, FIG. 1B shows a signal constellation tilting due to phase deviation between the transmitter and receiver ends, and FIG. 1C shows a signal constellation rotating due to additional frequency deviation between the transmitter and receiver ends. When the signal constellation of the demodulated signals is distorted as shown in FIG. 1B or 1C, the demodulated signals cannot be properly decoded to realize the original information Therefore, efforts have been made to solve these problems.
Please refer to FIG. 2 which is a functional block diagram of a conventional QAM receiver The received RF signals are converted into IF (Intermediate Frequency) signals by a tuner 11, and then sampled and digitized by an analog-to-digital (A/D) converter 12 with a sampling interval T. The digitized IF samples are further converted into baseband by a voltage controlled oscillator (VCO) 13, and then proper filtered to remove undesired high frequency components. Thus the received signals are demodulated. However, when the phase offset exists between the central frequency of the VCO 13 and that of the carrier at IF, a cross talk occurs between the in-phase channel and the quadrature channel of the basedband signal. A carrier recovery apparatus 14 is used to estimate the Δθ[n] so that the VCO 13 can adjust its phase according to such carrier information to eliminate the cross talk of the baseband samples. Such conventional carrier recovery method, for example that disclosed in U.S. Pat. Nos. 5,058,136, 5,519,356 or 5,940,450, provide parameters essentially relevant to phase deviation to modify the local oscillator at the receiver end so as to eliminate the phase deviation. If any frequency deviation exists, however, more parameters and modifying steps will be required to work on the phase deviation in order to eliminate both of the phase and frequency deviation problems. Therefore, it will take a lot of time to complete the modification and converge the system. Further, it is difficult to balance the modifying rate and the resulting precision.